The invention relates to apparatuses for wrapping and sealing articles in a folded thermoplastic film. More specifically, the invention is directed to a mechanism for cutting and sealing the side edges of the plies of plastic enclosing the wrapped article.
There exist a wide variety of automated apparatuses, known as shrink-wrap machines, which provide a continuous supply of thermoplastic film for wrapping a series of articles fed through the machine. Machines of this type are well known in the art and are described generally in Stork U.S. Pat. No. 5,956,931. Typically, a shrink-wrap machine will include a conveyor system for transporting the objects to be packaged through the machine in serial fashion. Thermoplastic film is continuously dispensed from a supply roll disposed in close proximity to the conveyor belt. The film is usually dispensed as a single sheet, and the machine has mechanisms for folding the sheet to form two plies, placing the sheet onto the conveyor system, and depositing the article for packaging between the plies of the folded sheet. The conveyor system sends the film-draped articles on to further mechanisms in the machine where cutters and sealers cut and seal the plastic around the article, thereby enclosing it within the film to form the finished, packaged product. Typically, the cuts and seals are effected by subjecting the thermoplastic sheet to a heated knife bar or a heated sealing wheel. A knife bar is used to make transverse cuts, such as those for creating the end seals between individual packaged articles carried along the conveyor system. The shrink-wrap machine is usually automated through software programs that control the speed of the conveyor system, the timing of the stroke of the knife bar, and the height of the cutters, all of which function to control the dimension of the finished package.
The open side edges of the folded thermoplastic sheet are sealed together as the sheet is transported by the conveyor system. A heated sealing wheel is optimally used to make the side seal because it can effect a continuous seal and is not dependent upon the dimensions of the final sealed package. To ensure that the plastic sheet is driven through the sealing wheel at a constant uniform rate and that it is fed straightly and evenly, certain mechanisms such as rollers and grip chains are employed to grasp and urge the plastic sheet along. These mechanisms must be placed in close proximity to the heating wheel in order to be effective. However, the available space adjacent to the heating wheel is limited and these grabbing mechanisms are usually required to be substantially offset from the sealing point. Thus, in order for the plastic sheet to be engaged by these grabbing mechanisms, a substantial extra amount of the plastic sheet edge is required to be extended beyond the sealing edge of the heating wheel. This has always resulted in excess waste of thermoplastic material. It would be desirable to eliminate this waste of material, and provide a means for decreasing the amount of thermoplastic sheet edge material that must extend laterally beyond the heating wheel when effecting a side seal in the shrink-wrap process.